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An increasingly useful type of bomb being used by U.S. and British air forces is bomb that (other than being painted blue) looks, flies, and is guided just like a large conventional bomb, but contains no explosive at all. Inside its standard steel jacket is nothing but plain, everyday, inert concrete.

Concrete-filled bombs have long been used by air forces around the world as practice bombs. After all, the goal of bombing practice is to test accuracy, not to make huge craters in the ground, and for that purpose concrete-filled bombs (usually with a device at the nose to make a big puff of smoke upon impact) are not only much cheaper but also much safer than bombs full of explosive material.

And in an even newer development, bomb designs are being evaluated for bombs that can be set for variable yields, adjusted in flight to deliver only as much "bang" as needed for the specific target. In current bomb tech, that's the leading edge.

More and more...

Aerial bombing came of age in World War One with small, slow biplanes and bombs small enough to be dropped by hand. But from the outset the drive was on to develop bigger and bigger bombs. Not only would a bigger bomb cause more destruction to the enemy but, given the poor accuracy of aerial bombing at the time, the bigger the blast radius the greater the chance the bomb would actually have some useful effect.

[Image3] In World War II typical bombs were 250 lbs, though some larger bombs, such as the 12000-lb (5400 kg) British "Tallboy" and 22000-lb (10 000 kg) Grand Slam were used against hardened strategic targets. By the time of the Vietnam war 500-lb (225-kg) and 1000-lb (450 kg) bombs were common. In today's warplanes fairly large 1000-lb and 2000-lb bombs are most common.

... then less and less...

But since the Vietnam war computerized flight controls, laser and GPS targeting systems and "smart" bombs have yielded a vast increase in bombing accuracy. That, in turn, has resulted in less and less need for huge bombs. After all, if super-accurate targeting and guidance systems can put the bomb within a few feet of its intended target, a large bomb becomes undesirable — all that explosive power is not needed to destroy the target, and in urban settings it has the very undesired effect of destroying much more than the target. Why risk levelling a city block and killing innocent people to get one enemy gun emplacement on the roof of one house?

So the direction of development of new bombs has, paradoxically, gone in two very different directions:

[Image1] Some are getting bigger, or at least stronger, but with smaller explosive charges. With the growing use of hardened underground bunkers by militaristic totalitarian governments — Saddam Hussein built dozens of them, as have Iran and North Korea — Western air forces have taken an interest in developing bombs capable of penetrating far underground through hardened defenses before exploding.

One such bomb, the GBU-28 "bunker buster," uses for its casing the thick, heavy steel barrel from an 8-inch howitzer for the ultimate in strength. So when it slams into the ground or the top of a bunker at near-supersonic speed, on inertia alone its massive hardened-steel casing is capable of punching through 60 feet (18m) of earth or 20 feet (6m) of hard concrete before detonating its relatively small explosive charge (630 lbs out of a total bomb weight of 5000 lbs).

[Image2]But in the other direction, lighter and smaller bombs are being developed for use against small targets, particularly in urban areas where the intent is to minimize damage beyond the target itself.

This has resulted in new devices such as the U.S. Air Force's GBU-39 Small Diameter Bomb, a physically compact bomb that allows a fighter or bomber to carry four times the usual number of bombs. Its compact size and 285-lb weight harken back to the small bombs used in World War Two. Compared to other current bombs, this small bomb delivers a lot less bang, but delivers it very, very accurately and its blast radius is relatively quite contained.

... then no bang at all

But what could be more "blast-contained" than at bomb that doesn't explode at all?

After the invasion of Iraq and subsequent years of urban fighting where Iraqi insurgents often deliberately set up gun enplacements in populated areas, the use of non-explosive concrete bombs was expanded for its ability to take out single structures without destroying those nearby.

Concrete bombs use all the same high-accuracy delivery and guidance systems as other modern bombs, including laser and GPS guidance to allow them to strike with great accuracy. For pilots the bombs are operated and used just like conventional bombs, they just carry no explosive. They are painted blue so that if the bombs are later found on the ground it'll be obvious they're not explosive.

That's not to say concrete bombs merely go "thud."

A half-ton or full ton of steel and concrete falling at near-supersonic speed represents a huge amount of energy, and all that energy is instantly dissipated when the bomb hits a structure, vehicle, or the ground. In practice military observers have reported that a concrete bomb striking a house will completely level the house and destroy anything within. But unlike with a conventional bomb, surrounding buildings will usually receive little damage.

This somewhat lighthearted video from the U.K.'s National Geographic Channel illustrates the difference between explosive and concrete bombs in an unscientific but visually effective way.

And finally: dial-a-bang and the "safer" bomb

The latest development in matching the explosion to the target is a bomb which can be "tuned" to deliver only the precise amount of explosion required. That is the goal of a new project by the U.S. Navy, which just put out a request for ideas for a bomb that can be be set, just before it's dropped, to deliver only the amount of explosive power the pilot or weapons officer deems necessary for the given target.

Can it be done? No one yet knows, but the Navy is willing to spend at least $9.9 million to find out. For anyone with a good idea, the Navy's checkbook is ready.